In recent years, a spectrum analyzer has been known as a measurement device that displays the distribution (spectrum) of a frequency component included in a radio frequency signal, which is a measurement target, and analyzes the measurement signal. The spectrum analyzers include a sweep type and a fast Fourier transform (FFT) type.
For example, Patent Document 1 discloses a signal analysis apparatus as the sweep spectrum analyzer. The signal analysis apparatus disclosed in Patent Document 1 includes, for example, a sweep signal generation unit, a resolution bandwidth (RBW) filter, a logarithmic converter, and a video filter.
For example, a spectrum analyzer having the structure shown in FIG. 7 has been known as the FFT spectrum analyzer. The spectrum analyzer shown in FIG. 7 includes a frequency conversion unit 110, a variable attenuator (V-ATT) 101, an analog/digital converter (ADC) 103, a frequency response (f-response) correction filter 104, a signal analysis unit 105, and a display unit 106. The frequency conversion unit 110 includes a step attenuator (S-ATT) 111, a mixer 112, a BPF 113, and an intermediate frequency (IF) amplifier 114.
In this structure, the S-ATT 111 adjusts the step of a radio frequency signal to attenuate the radio frequency signal such that the radio frequency signal is input as an analysis target to the mixer 112 at the optimal level. The gain of the V-ATT 101 is set to a predetermined value corresponding to a predetermined reference level (reference level) such that the ADC 103 does not overflow and the V-ATT 101 attenuates the input signal.